Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube reactions of CHF₃: comparison of product branching ratios§

The threshold photoelectron and threshold photoelectron photoion coincidence spectra of CHF\(_3\) in the range 13.5 – 24.5 eV have been recorded. Ion yields and branching ratios have been determined for the three fragments CF\(_3^+\), CHF2\(^+\) and CF\(^+\). The mean kinetic energy releases into fragment ions involving either C-H or C-F bond cleavage have been measured, and compared with statistical and impulsive models. CHF\(_3^+\) behaves in a non-statistical manner characteristic of the small-molecule limit, with the ground electronic state and low-lying excited states of CHF\(_3^+\) being largely repulsive along the C-H and C-F coordinates, respectively. The rate coefficients and product ion branching ratios have been measured at 298 K in a selected ion flow tube for the reactions of CHF\(_3\) with a large number of gas-phase cations whose recombination energies span the range 6.3 through 21.6 eV. A comparison between the branching ratios from the two experiments, together with an analysis of the threshold photoelectron spectrum of CHF\(_3\), shows that long-range charge transfer probably occurs for the Ar\(^+\) and F\(^+\) atomic ions whose recombination energies lie above ca. 15 eV. Below this energy, the mechanism involves a combination of short-range charge transfer and chemical reactions involving a transition state intermediate.